A number of companies, such as IBM and Packard-Hughes, produce high density flexible electrical cables, often referred to as "flex cable", for use in routing electronic data. The flex cables pack a large number of separate metal conductors, the signal lines, that are insulated from one another within a relatively thin flat web of limited width. In appearance the cable resembles a thick stiff belt. The cable may be bent around corners or wrapped, much like an ordinary leather belt.
To meet the growing number of inputs and outputs required for microelectronic circuits and packages, those and other companies have designed and fabricated high density flexible cables containing 125 signal lines or greater per inch of width. The cables incorporate a polyimide film as a substrate for lithographically defined electrical conductors or, as variously termed, signal lines. The majority of flex cables currently used in electronic packaging are not shielded; that is the cables do not include metallic ground planes or shields located in close proximity to the signal lines. Although such unshielded lines are suitable for signals at frequencies of ten MegaHertz or less, at higher frequencies, the signals suffer unacceptable power loss.
By incorporating shields and/or ground planes within the cable, flex cables are ideally suited for routing signals at high frequencies or rates at up to 10 Gigahertz and greater. Two forms of flex cables are available, a microstrip configuration, such as from Tektronix company, and a strip line configuration. Both configuration will carry signals without significant power loss, but the stripline configuration suffers less power loss than the microstrip configuration.
In the stripline configuration the substrate containing the signal lines is sandwiched in-between two metal layers, the ground planes, and are separated therefrom by insulating films, such as polyimide film used for the substrate, a strong flexible plastic electrical insulating material. As brought out in the description that follows in this specification that electrically insulating material also is found to possess favorable thermal characteristics that enables the practice of the invention. In practice, the flex cables also frequently include an outer insulating wrap, also formed of a polyimide film, to the foregoing configuration. In contrast, the microstrip configuration of the flex cable contains only a single ground plane layer.
Due to the multi-layer structure of the flex cable, it becomes difficult to properly attach the cable ends to external electrical components or circuits, such as found on a circuit board or other substrate. A typical application requires the cable's signal lines to be soldered to appropriate contacts on a printed circuit board, with those contacts typically being organized in a rectangular array formed by one or more rows of contacts. With 125 lines packed into a one inch width, soldering connections poses somewhat of a challenge. One practice of meeting the high density of signal lines is to fan out the signal lines to more widely displaced contacts on the circuit board, a practice that requires a greater area for the cable termination on the circuit board.
Further, both of the stripline cable's ground planes require connection to ground connections on the printed circuit board. Those ground plane connections must be made in the immediate vicinity of the location on the circuit board at which the signal lines are attached. If the ground connections are not so positioned, the ground current paths effectively introduce large inductances within the signal line circuits, thereby degrading the cable's electrical properties, more particularly, limiting the upper range of frequencies for which the cable is suited. Packard-Hughes company is known to market a shielded flex cable containing only a single ground plane. Because the ground plane in the Packard-Hughes cable is connected to the circuit board at a location distant from the connection of the signal lines, that cable cannot be regarded as either a microstrip or a stripline version, but is somewhat of a hybrid structure. Because of the distant ground connection, it is believed to suffer from degraded high frequency performance.
The attachment of flex cables containing fewer number of leads than the 125 lines per inch and above to corresponding solder pads on printed circuit boards has in the past been accomplished by exposing the ends of the signal lines, pre-tinning them with solder, and soldering them, simultaneously, to the respective solder pads on the circuit board. One known apparatus for simultaneously soldering multiple electrical leads of an electric cable to corresponding multiple cable contacts, solder pads, on printed circuit board is the hot bar station, a soldering apparatus marketed by the Uni-Tek Company of Monorovia, Calif.
That soldering apparatus contains a movable heated rectangular metal bar, called a hot bar. For soldering a longitudinal edge of the hot bar is brought into contact with the cable's exposed tinned copper leads. The hot bar is carried by a piston mechanism in the station, which is used to raise or lower the bar. Electrical leads from the station's power supply connect the front and back surfaces of the hot bar in a current conducting path to the power supply.
High levels of DC current through the metal bar, which possesses electrical resistance, produces I.sup.2 R losses in the bar, creating the necessary heat for soldering. The generated heat is conducted to the edge of the hot bar and produces a temperature along that edge that is high enough to at least exceed the eutectic temperature of the solder being used to tin the cable leads and solder pads. As one appreciates the hot bar station is much like an ordinary electrical soldering iron in function and in principal, but performs the soldering function in locations and circumstances in which the soldering iron is impractical. The present invention also takes advantage of a hot bar station.
In typical operation, the exposed tinned copper leads of the cable are placed in contact with the corresponding solder pads, also tinned with solder, located on the printed circuit board. The two are clamped in that relationship on the bed of the hot bar station, which also serves as a heat sink. The electrically heated hot bar is lowered and its edge is placed in direct physical contact with the multiple exposed tinned leads, raising the latter to the eutectic temperature at which the solder reflows. Heat is also conducted from the tinned leads to the solder pads and the latter are also heated to the temperature at which the solder on those pads reflows. When the heating is halted, either by turning off the current to the hot bar or by withdrawing the hot bar, the liquified solder cools and re-solidifies, resulting in a soldered connection between the electrical leads and the corresponding solder pads on the circuit board.
Accordingly, an object of the present invention is to apply the hot bar station to a technique for soldering shielded high density stripline flex cables to external terminations, quickly and easily.
A further object of the invention is to provide a technique for soldering insulated electrical leads to solder pads on associated apparatus without requiring the heating source to have direct physical contact with either the leads or the solder pads.
A still further object of the invention is to provide a technique for simultaneously soldering large number of signal lines, 125 and greater, to corresponding terminations on a circuit board without requiring the signal lines to be fanned out.
And an ancillary object of the invention is to provide a new end configuration to a shielded high density stripline flex cable; and a new structure for a cable termination that mates with the end configuration and defines a new cable to circuit board attachment system.